The invention relates to apparatus for the control of the fuel-air mixture of an internal combustion engine, in particular fuel enrichment during engine warm-up. The apparatus is further capable of switching over the fuel-air mixture control system on the basis of engine temperature and other operational conditions. The type of apparatus to which this invention particularly relates is an electronic fuel injection system wherein the injected fuel quantity per stroke is determined by circuitry containing an energy storage device, for example a capacitor, which is charged and discharged in controlled manner and thereby determines the duration of fuel injection control pulses. A temperature transducer is suitably provided in the vicinity of the engine, for example in the cooling system.
It is well known that the operation of internal combustion engines which are cold, i.e., which have not reached their normal operating temperature, requires a fuel-air mixture which is enriched in fuel, i.e., which contains more combustible fuel than is required for a stoichiometric mixture. Furthermore, the amount of fuel may exceed the power delivered by the engine during the warm-up phase.
The reason for having to supply excess fuel is that during and after the starting of a cold engine, a substantial fraction of the fuel supplied to the mixture condenses on the walls of the cylinders and the induction manifold and temporarily does not participate in the combustion process. Under certain circumstances, the raw fuel may in fact drain into the oil sump. Furthermore, a substantial amount of power is used for heating up the cold walls of the cylinder and the power lost to friction is also increased during the warm-up phase of operation.
Generally speaking, it may be said that the warm-up operation of an engine is defined by a multitude of factors and is a fairly complicated process which, furthermore, varies in each type of engine and for various manners of operation. Thus, the warm-up process must be so performed as to maintain smooth and reliable engine operation without causing the engine to jerk or stall, which implies that the control of the warm-up enrichment process, which may under certain circumstances require an enrichment as high as four times the normal amount of fuel, must take place in a very sensitive manner capable of adaptation to various conditions.